Polyetherimides comprising the reaction products of a dianhydride of a diphenolic compound and a diamine, e.g., methylenedianiline are known to form useful resinous coatings for electrical conductors. See, for example, Heath and Wirth, U.S. Pat. No. 3,847,867. Furthermore, polyamideimides comprising the reaction products of a carboxylic acid anhydride, a diamine and a diisocyanate are known to form useful coatings for electrical conductors. See, for example, Pauze and Holub, U.S. Pat. No. 3,817,926. In addition, polyamideimide resins comprising the reaction products of a tribasic acid anhydride and a diisocyanate are known to form useful insulating coatings from Nakano and Koyama, U.S. Pat. No. 3,541,038. Moreover, it is disclosed in Y. G. L. Lee's copending, commonly filed U.S. patent applicaton Ser. No. 137,991, filed Apr. 7, 1980 now abandoned, assigned to the assignee herein, that useful insulating coatings comprise the polycondensation product of a dianhydride of a diphenolic compound and a diisocyanate. The contents of the foregoing patents and application are incorporated herein by reference.
It has now been discovered that uniquely useful reaction products comprising polyetheramideimide resins can be formed from the reaction of (A) a combination of a dianhydride of a diphenolic compound and a tribasic acid anhydride and (B) a diisocyanate and/or a diamine. Such products are preferred for use as insulating coatings for electrical conductors, e.g., magnet wire and magnet strip, because they are safer, better and more economical. The resins are preferably applied as compositions diluted in an organic solvent.